Now pressure is growing to fight obesity, particularly among children, through social change like taxing sugary drinks and removing vending machines from schools.

Will it work? No one knows. But the fact that unproved strategies are moving ahead anyway absolutely infuriates Barbara Hansen, a pioneering obesity researcher at the University of South Florida. She says the time has come to stop trying things that sound good, and stick to the evidence. Otherwise, we could do more harm than good.

"I'm concerned about childhood obesity, but I'm much more concerned about our attempts to cure it," she said.

"We have been blaming teachers. We have been blaming parents. We have been blaming the children.

"And yet, we don't have good things for them to do."

Hansen has evidence on what works, and what doesn't. The proof is in her monkeys.

Monkeys metabolize food almost exactly as we do, but they're better research subjects. They can't cheat on Weight Watchers. They aren't tempted by drive-through french fries.

For decades, Hansen has fed a colony of rhesus monkeys the ultimate heart-healthy diet, low in fat, with almost zero cholesterol.

Still, some monkeys grow fatter each year, while others stay lean.

Just like us.

The body knows, but how does it know?

As a graduate student, Hansen was drawn to obesity research by this question: How do people know when to stop eating?

"Feeding regulation is one of the great scientific mysteries," Hansen said on a sunny afternoon in her office.

Hansen, 68, wears her blond hair scraped back in a bun, a clinical effect softened by the gentle smile that spreads across her face as she explained the inner workings of a well-tuned body.

"Like an extraordinary symphony, our bodies know when to make our heart rate faster, our skin dilate, our gastrointestinal system feel full, our brain feel satiated. We take in a couple of tons of food each year, and we may gain one pound."

She is talking, of course, about the body's natural adjustments, not the regimen we try to inflict upon it through diets.

"Somehow, your body does know how fat you are. To this day, we don't know how the body is figuring that out."

Hansen, who has a Ph.D. in physiology and psychology and directs USF's Obesity, Diabetes and Aging Research Center, has spent her entire career looking for answers — first in humans, then in monkeys. Her 53-page resume (PDF) is filled with academic articles and honors for her work.

In her research and her teaching, one theme resounds: Popular beliefs about fat are often flat wrong.

"Every one of us thinks of an obese person as a glutton, a person who's unable to control themselves, who probably sneaks into the other room and grabs ice cream and potato chips," Hansen said as a guest lecturer to a graduate class of USF students last year. "In fact, that is a very rare event."

Obese people, she pointed out, generally eat the number of calories they need to maintain their current weight, like thin people do. That, she says, is not overeating.

But if gluttony isn't the cause of obesity, what is?

Hansen points to genetics as the reason why so many people work so hard at losing weight, only to gain it back.

Then why are so many people so much heavier than their grandparents were? Simply because in our society, Hansen and other experts say, calories are so freely available, and physical demands are far less. From cars to whopping portion sizes, modern life allows our genes to express themselves as never before.

"Just because I have a genetic propensity to something doesn't mean it's necessarily going to be expressed unless I put it in the appropriate environment."

Hansen believes she has demonstrated the significance of genetics in her monkeys.

Manipulating diets to see what happens

Since 1973, Hansen has been studying a colony of rhesus monkeys, receiving more than $13 million in cumulative funding from the National Institutes of Health, whose support has been essential to her work.

Her monkeys, which are housed off-campus, live in large cages that sit in white-walled, brightly lit rooms.

In her most famous experiment, she kept one group of monkeys on a strict, meticulously documented diet from their early adulthood until death. The other group had unlimited access to healthy food.

In the group that could eat freely, some monkeys grew plump, while others stayed thin. Some developed obesity and diabetes; others did not.

In the calorie-restricted group, not a single monkey developed obesity or diabetes.

Hansen believes the implications are tremendous — strict calorie limits could prevent obesity and diabetes. And in the absence of such food restrictions, genetics kick in.

To drive home the point, Hansen cites another experiment.

In this one, she placed feeding tubes into the bellies of several monkeys. She infused Ensure, the human nutritional supplement, directly into their stomachs. In a classic example, the monkey didn't know it, but she was making him overeat by about 50 percent of his normal caloric intake.

The monkey still got his regular meals, taken by mouth. But after a few days, he refused food.

The tube-feeding continued until the monkey had gained about 30 percent of his normal body weight. Then Hansen cut it out completely.

For the next 20 days, the monkey refused all food. Only after his body weight had returned to nearly normal did the monkey begin eating again.

On both sides of the equation — overeating and under­eating — the studies demonstrated to Hansen how well the animal's own regulatory system works — and that tampering with it is no easy undertaking.

The media message is mostly messed up

If you really want to get Hansen going, mention some new diet you've just read about. A Parade article that claimed you could lose lots of weight with simple little adjustments had Hansen scrawling exasperated comments in the margins.

By eliminating 1 cup of starch every night and doubling up on non-starchy veggies, you could save more than 80,000 calories and drop almost 23 pounds in one year!

Even diet soda and artificially sweetened foods can lead to weight gain.

"False. Silly."

She collects examples like these to use in her lectures to students. One of her favorite props is a worn copy of Time magazine, featuring a fresh-faced boy with a bulging tummy eyeing a double scoop of ice cream. The headline: "Our Super-sized Kids."

What she sees as the article's implication — that the kid is to blame for his fat — gives Hansen fits. Her fear is that we're raising generations of kids — not to mention their parents and teachers — to think that unless they can control their weight, they are failures.

"The message about obesity that I like least in today's media is, 'You did this to yourself, and you can fix it!' Neither is, in my view, accurate for the vast majority of overweight persons," Hansen explained in Obesity, a new book written by researchers to which she contributed.

Let science drive any institutional change

Hansen understands the urge to act on concepts that sound good, like removing vending machines from schools. But she believes such ideas should be studied to see if they work before they are enacted.

"My biggest concern is armchair science," said Hansen. Her belief in genetics' key role does not mean she's against seeking effective public health measures, she stressed.

"It's really the science and the question is, 'What does the science show?' "

Still, many in the public health community feel just as strongly that the obesity epidemic demands immediate action based on the best information available.

Kessler, who himself battles a weight problem, agrees with Hansen that obesity isn't a simple problem of willpower. Yet he doesn't see genetics as the primary driver in obesity.

Kessler believes our minds are wired at a very young age to respond to the stimulus provided by food, especially items laden with salt, sugar and fat.

And that, he says, is why he thinks it will take changing social norms and personal attitudes to overcome the conditioned behavior of overeating.

The lure of today's food products is one reason he says animal research may be of limited value in unlocking the mysteries of obesity.

"You put in the cage chocolate chips and peanut butter and crackers and you will see a much greater weight gain than when you feed just animal chow," he said.

That has been seen in some rodent studies, Hansen acknowledges. So she has experimented similarly. For six weeks at a time, her research team would serve up yummy foods dense in calories — like cookies, bananas, gummy bears, Brazilian nuts — to monkeys of different weights and ages, who also got their standard monkey chow.

The monkeys enjoyed many of the new foods, she found. But they ate less monkey chow to compensate. "We were not able, through any of our ingenuity, to produce sustained overeating in a monkey," Hansen said.

Eat more and gain, eat less to lose weight

On this much, the science is clear: No fad diet or public policy change can beat a basic law of thermodynamics. Eat more calories than you use, and you will gain weight.

And it's terribly difficult to lose weight through exercise alone, given that it can take a 150-pound person an hour of brisk walking to burn a 300-calorie candy bar.

"If you decide to attempt to lose weight, the No. 1 step has to be to constrain and restrain calories — continuously for good," Hansen says. "And that's very difficult."

She says this not to be discouraging — as much as anyone, Hansen knows the health risks of obesity. But until people truly understand how tough it is to lose weight and keep it off, they'll only grow discouraged trying approaches that do not work.

Her best advice: Weigh yourself regularly. If you're overeating, that number will rise, telling you that you need to cut back.

If you believe popular diet advice, much of which holds you should eat most of your calories early in the day, Hansen shouldn't be thin. She eats a light breakfast and skips lunch. She prefers to eat a complete dinner, with appetizers, a full plate of food and wine. She rarely eats dessert.

She follows her own advice, weighing herself four to five times a week and adjusting what she eats if the dial edges up.

But Hansen credits heredity, not iron will, for her slim figure. She can look back on both sides of her family tree and not find obese ancestors.

One day, she hopes drugs or devices will make it easier for people to lose weight and keep it off. So far, she noted, the medications available have made only modest impact on people's weight.

Hansen thinks bariatric surgery is one of most effective treatments available today, although it's only an option for the very obese. She's testing out some promising new therapies on the monkeys, work that is funded by industry.

But her research tells her that appetite resists long-term correction.

Several times, she took monkeys whose calories were restricted and let them eat all they liked. Even after 20 years, the monkeys on the restrictive diet were still hungry.